1. Field Of The Invention
The present invention relates to the rheological testing to measure fluid properties such as for controlling the base gel quality during well fracturing treatments, and more particularly, to the measurement of these fluid properties using a helical screw rheometer controlled by a computer means.
2. Description Of The Prior Art
Once an oil or gas well is drilled, it is sometimes necessary to fracture a formation to improve the flowability of the hydrocarbons trapped in the formation. To perform a fracturing job, specialized fracturing fluids suitable for the specific job are blended. The fracturing fluids are delivered to the well site by blending selected chemicals into a base gel, the characteristics of which can be controlled based upon rheological properties as determined from real-time measurements taken, for example, through the use of a laminar flow rheology loop. Such measurements can also be used for predicting friction pressure loss and bottom hole treating pressures that are likely to occur when the blended fluid is pumped down hole during the fracturing process. Such control and prediction abilities enhance the chances of obtaining a good result from the fracturing job under way, and the information obtained therefrom is helpful in designing other blends for subsequent jobs.
One technique for obtaining such control and prediction abilities is based upon the shear rate versus shear stress relationship applicable to the particular base gel being used. This relationship can be determined by monitoring differential pressures on various lengths and diameters of the flowing base gel. Measurements of such differential pressures correlate to points on a graphical representation of the particular shear rate versus shear stress relationship. For example, a least squares fit calculation can be used with the differential pressure measurements to compute a straight line having a slope representing the n' value and having a unity intercept representing the k' value for the log of the shear rate versus the log of the shear stress relationship. These values are used by a computer for correlating the turbulent flow in pipes of other diameters and for providing other suitable information on a realtime basis. The mathematical relationships of this technique are known in the art.
The foregoing technique of using differential pressure measurements to develop a shear rate versus shear stress relationship can be correlated into useful, real-time information by which the quality of a gel can be controlled and by which predictions of downhole phenomena can be made has been implemented by a portable laminar flow rheology skid developed and used by Halliburton Services prior to October, 1984. This skid is transportable by a vehicle dedicated to that function, such as a pickup truck, but is electrically connectable to a COMPUVAN.RTM. testing vehicle for providing the differential pressure information to computers in the testing vehicle. The computers generate the correlations and predictions based on the differential pressure information
In operation, this prior skid has been used with a blending system that includes frac tanks containing the base gel, a blender tub, a suction pump for pumping the gel from the frac tanks to the blender tub, a differential pump for pumping the blend from the tub for subsequent high pressure pumping, by other pumps, into the well Additives can be blended into the base gel through application into the tub or along the flow path as known in the art. The prior laminar flow rheology loop has an inlet connected to receive a portion of the clean base gel tapped from the flow between the suction pump and the tub. The rheology loop has an outlet through which the tapped portion flows into the tub.
The tapped flow circulates through at least part of the loop which comprises four test pipe sections having different diameters (e.g., nominal inner diameters of 3/8 inch, 1/2 inch, 3/4 inch and 1 inch). A respective differential pressure transducer and two respective pressure taps are used to measure the pressure drop over a specified length in each pipe section. To increase the range of gel viscosities than can be accommodated in this flow loop, only three test pipe sections are used by the computer program at any one time (in this embodiment, either the three larger pipe sections or the three smaller pipe sections). If a relatively thin fluid is being used, for example, then lower flow rates or larger pipes will keep the flow laminar.
In this prior rheology loop, the flow rate of the base gel being circulated through the loop must also be controlled and measured. The flow rate is controlled by a 1-inch ball valve, and the flow rate is measured in one of two different ways, depending upon the type of gel being circulated. If an aqueous (conductive) gel is measured through the loop, a magnetic flow meter measures the flow rate. If a non-aqueous (non-conductive) gel is circulated, a standard turbine flow meter measures the flow rate.
The prior skid also includes a pH probe and temperature probe placed in the flow stream to monitor these characteristics of the base gel.
The prior skid also includes a bypass valve so that the 3/8-inch test pipe section can be removed from the flow stream. This is used not only to select between which set of three test pipe sections is to be used, but also to bypass the smallest pipe should the outlet pressure of the blender tub become marginal whereby such bypassing decreases the back pressure that the laminar flow loop creates in the portion of the flow taken from the blender tub.
Although this prior skid functions satisfactorily, an improved rheology test system has been constructed so that a rheology flow loop skid thereof can be conveniently and readily transported on a complementally constructed vehicle also capable of carrying the computational equipment needed to perform the necessary calculations. This prior art apparatus for measuring rheological properties, disclosed in U. S. Pat. No. 4,700,467 to Frey et al. and assigned to the assignee of the present invention, provides a skid-mounted laminar flow rheology loop which is more compact and lighter in weight than the previous rheology skid so that it can be carried on a complementally constructed test vehicle also carrying computational equipment forming another part of the overall system.
The apparatus of U.S. Pat. No. 4,700,567 provides a transportable rheology test system for generating, at the location of a well, predictions of friction pressure loss in bottom hole treating pressure during a fracturing system being performed on the well. The system comprises a self-propelled vehicle for moving the system to a well site, computer means mounted in the vehicle for calculating, in response to differential pressure values, n' and k' parameters of a shear rate versus shear stress relationship of a flowing substance, from which parameters the predictions are generated. This apparatus further comprises rheology loop means for detecting a plurality of differential pressure values along a plurality of lengths of a fluid flowing therethrough and for providing to the computer means signals representing the plurality of differential pressure values. The rheology flow loop means includes a base and a helical pipe assembly mounted on the base.
The apparatus of U.S. Pat. No. 4,700,567 performs satisfactorily, but requires the rheology flow loop described as well as a plurality of differential pressure gauges associated therewith.
There is, accordingly, a need for rheology test equipment which eliminates much of this piping and simplifies the number of pressure connections. The present invention meets this need by substituting a helical screw rheometer for the previous rheology loop and uses only one pressure transducer located at the outlet of the rheometer.
Generally, the helical screw rheometer is a known device for measuring fluid properties and was developed by Sandia National Laboratories, originally for on-line measurements of slurries in coal liquefaction processes. The accuracy of helical screw rheometers and the ease with which they may be used to determine fluid properties makes them desirable for use at a well. However, prior helical screw rheometers have essentially been laboratory devices, and there is a need for a field version, particularly one designed to control base gel quality during well fracturing treatments. The present invention addresses this need by providing a field helical screw rheometer which is controlled by a computer means.